U.S. Pat. No. 6,712,029 discloses a two-stroke engine wherein substantially fuel-free air is supplied from an air channel to the transfer channels via a piston window. In the air channel, a throttle flap is provided for controlling the supplied air quantity. The throttle flap is completely or at least substantially closed during idle and during low engine load. An underpressure is formed in the air channel section downstream of the throttle flap because of the substantially closed throttle flap. The underpressure causes ambient air or air from the air channel section upstream of the throttle flap to be drawn by suction through leaks in the channel. A reliable sealing of the air channel can be achieved only with great complexity for a throttle flap which is substantially closed.